Flame-melting is a process whereby powders are fed into a device that uses high temperature to transform the particles into crystalline and/or amorphous materials. Powders are fed into the flame-melting device by powder feeders such as vibratory, screw, twin-screw, drum, and gas suspension feeders. These feeders can be used to deliver powder feed stock to the flame-melting apparatus at a controlled rate. Feed rate is typically controlled volumetrically or by a mass loss. In the feeder types currently used, when feeding fine powders that have less than ideal flow properties, difficulties are encountered. First, transporting the powder in a continuous, stable manner without bridging (as described in the following paragraph) is a concern. Second, supplying the powders within a given size or particulate range without significant agglomeration is also a concern.
In many instances, when feeding powders (i.e., powders that lack free-flow properties) through a feeder, such as a screw or a vibratory feeder, the powders may pack to a high density and form a packed bed that further limits the transport of powders. Such packing typically results in a situation where the powder is removed from the powder bed where the screw makes physical contact with the powder bed, but the remainder of the powder remains packed. This packing forms a structure that inhibits flow (i.e., “bridging” occurs). When bridging occurs, either the powder transport ceases, or it becomes intermittent (when pieces of this dense pack break, there is powder transport). This is a highly undesirable situation.
Several of the feeders known today address the first problem by using a combination of various powder transport techniques. For example, a screw-type feeder may be placed on a vibrating platform or include a mechanical mixer within the powder feed container. However, there is still further a need to improve the problem of agglomeration of the powder being fed. Further, for example, although gas atomization aids in reducing agglomeration, the gas flow that accompanies the powder is not always desirable, as the carrier gas flow may interfere with the rest of the process. Furthermore, for example, gas suspension feeders are not typically capable of feeding powders at high rates desired for some processes.
Vibratory feeders tend to have the powders agglomerate into larger than desired sizes, and the feed rate pulsates. Disadvantages of vibratory feeders include, powders agglomerating into undesirable larger sizes, as well as undesirable variable feed rates.
One undesirable characteristic of gas suspension powder delivery systems is that the use of excess carrier gas, an undesired byproduct of the powder feed, tends to cool the flame temperature. This is because a relatively high pressure of gas (i.e., a relatively high carrier gas flow rate) is needed to suspend and carry the powder particles. The result is a relatively low powder concentration in the gas. In addition, there is a practical limit on the amount of powder that can be suspended in the carrier gas, typically depending on the density and particle size of the powder. Therefore, feeders based on gas atomization are generally limited to relatively lower rates of powder feed. Further, the gas used to feed the powder may interfere with the melting of the powder by the flame. For example, the feed gas can mix with the combustion inputs, such as fuel and oxygen, and result in a significantly cooler flame temperature, which is undesirable. This can be especially problematic at higher feed rates.
Using brush feeders, powders tend to agglomerate into larger than desired sizes. Using screw, brush, and atomizer combinations, a screw inside a cylindrical feed shoot transports powder to the rotating brush. Powder is blown out the brush and entrained in gas flow. The disadvantage is that excess gas is needed to suspend the powder and is an undesired byproduct of the feed, and the extra gas can cool down the flame temperature.
Rotating drum feeders also have a problem with the tendency to have the powders agglomerate into larger than desired sizes, and powders often bridge on the rotating drum feeders.
There is a need in the art for a powder feeder that avoids bridging of nonflowing powders, clumping of powders being fed, is capable of breaking down large agglomerates, controls the size of the feed powder particles, while at the same time providing a uniform feed rate.